staging: iio: adc: ad7606: Add support for threaded irq

This patch replaces the use of a polling ring buffer with a threaded
interrupt.

Enabling the buffer sets the CONVST signal to high. When the rising edge
of the CONVST is applied, BUSY signal goes logic high and transitions low
at the end of the entire conversion process. The falling edge of the BUSY
signal triggers the interrupt.

ad7606_trigger_handler() is used as bottom half of the poll function.
It reads data from the device and stores it in the internal buffer.

Signed-off-by: Stefan Popa <stefan.popa@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>